U.S. patent application number 10/568786 was filed with the patent office on 2006-12-21 for semiconductor light emiting device and method of producing the same.
Invention is credited to Tsuyoshi Tojo, Shiro Uchida.
Application Number | 20060284186 10/568786 |
Document ID | / |
Family ID | 35510041 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060284186 |
Kind Code |
A1 |
Uchida; Shiro ; et
al. |
December 21, 2006 |
Semiconductor light emiting device and method of producing the
same
Abstract
To provide a semiconductor light emitting device capable of
improving an aspect ratio of a laser beam to make it close to a
circular shape and a method of producing the same, a first
conductive type first cladding layer 11, an active layer 12, and a
second conductive type second cladding layer 17 having a
ridge-shaped portion RD as a current narrowing structure are
stacked on a substrate 10; wherein the ridge-shaped portion
includes a first ridge-shaped layer 15 on the side close to said
active layer and having a high bandgap and a second ridge-shaped
layer 16 on the side distant from the active layer and having a low
bandgap, so that the semiconductor light emitting device is
obtained. By using an epitaxial growth method, a first cladding
layer, active layer and second conductive type second cladding
layer are formed by being stacked on the substrate, a part of the
second cladding layer is processed to be a ridge-shaped portion,
and the second cladding layer is formed, so that the portion to be
a ridge shape includes the first ridge-shaped layer and second
ridge-shaped layer.
Inventors: |
Uchida; Shiro; (Miyagi,
JP) ; Tojo; Tsuyoshi; (Miyagi, JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL LLP
P.O. BOX 061080
WACKER DRIVE STATION, SEARS TOWER
CHICAGO
IL
60606-1080
US
|
Family ID: |
35510041 |
Appl. No.: |
10/568786 |
Filed: |
June 15, 2005 |
PCT Filed: |
June 15, 2005 |
PCT NO: |
PCT/JP05/10928 |
371 Date: |
February 16, 2006 |
Current U.S.
Class: |
257/79 ;
372/46.01 |
Current CPC
Class: |
H01S 2301/185 20130101;
H01S 5/2231 20130101; H01S 5/22 20130101 |
Class at
Publication: |
257/079 ;
372/046.01 |
International
Class: |
H01S 5/00 20060101
H01S005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 2004 |
JP |
2004-181111 |
Claims
1. A semiconductor light emitting device, comprising: a substrate;
a first conductive type first cladding layer formed on said
substrate; an active layer formed on said first cladding layer; and
a second conductive type second cladding layer formed on said
active layer, a part thereof having a ridge-shaped portion as a
current narrowing structure; wherein said ridge-shaped portion of
said second cladding layer includes a first ridge-shaped layer on
the side close to said active layer and having a high bandgap and a
second ridge-shaped layer on the side distant from the active layer
and having a low bandgap.
2. A semiconductor light emitting device as set forth in claim 1,
wherein said first ridge-shaped layer and said second ridge-shaped
layer are a layer with a high aluminum composition ratio and a
layer with a low aluminum composition ratio, respectively.
3. A semiconductor light emitting device as set forth in claim 2,
wherein an aluminum composition ratio X1 of said first ridge-shaped
layer is 0.60.ltoreq.X1.ltoreq.0.70, and an aluminum composition
ratio X2 of said second ridge-shaped layer is X2.ltoreq.X1.
4. A semiconductor light emitting device as set forth in claim 2,
wherein an aluminum composition ratio X1 of said first ridge-shaped
layer is 0.70, and an aluminum composition ratio X2 of said second
ridge-shaped layer is 0.65.
5. A semiconductor light emitting device as set forth in claim 1,
wherein a film thickness of said first ridge-shaped layer is 50 to
400 nm.
6. A semiconductor light emitting device as set forth in claim 1,
wherein a sum of a film thickness of a portion excepting said
ridge-shaped portion of said second cladding layer and a film
thickness of said first ridge-shaped layer is 750 nm or
smaller.
7. A semiconductor light emitting device as set forth in claim 1,
wherein an etching stop layer is formed on a boundary face of a
portion excepting the ridge-shaped portion of said second cladding
layer and said first ridge-shaped layer.
8. A semiconductor light emitting device as set forth in claim 1,
wherein said first cladding layer, said active layer and said
second cladding layer are formed by an AlGaInP-based material.
9. A semiconductor light emitting device as set forth in claim 1,
wherein said first cladding layer, said active layer and said
second cladding layer are formed by an AlGaN-based material.
10. A semiconductor light emitting device as set forth in claim 1,
wherein said first ridge-shaped layer is formed by a layer having
an equal refractive index to that of a portion excepting said
ridge-shaped portion of said second cladding layer.
11. A semiconductor light emitting device as set forth in claim 1,
wherein said first ridge-shaped layer is formed by a layer having a
lower refractive index than that of a portion excepting said
ridge-shaped portion of said second cladding layer.
12. A semiconductor light emitting device as set forth in claim 11,
wherein an aluminum composition ratio of said portion excepting
said ridge-shaped portion of said second cladding layer is 0.68,
and an aluminum composition ratio of said first ridge-shaped layer
is 0.75 to 0.80.
13. A method of producing a semiconductor light emitting device,
including: a step of forming at least a first conductive type first
cladding layer, an active layer and a second conductive type second
cladding layer by stacking on a substrate by an epitaxial growth
method; and a step of processing a ridge-shaped portion as a
current narrowing structure at a part of said second cladding
layer; wherein, in the step of forming said second cladding layer,
a portion to be said ridge-shaped portion is formed to include a
first ridge-shaped layer on the side close to said active layer and
having a high bandgap and a second ridge-shaped layer on the side
distant from the active layer and having a low bandgap.
14. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, a layer having a high aluminum composition ratio
and a layer having a low aluminum composition ratio are formed as
said first ridge-shaped layer and said second ridge-shaped layer,
respectively.
15. A method of producing a semiconductor light emitting device as
set forth in claim 14, wherein in the step of forming said second
cladding layer, a layer having an aluminum composition ratio X1
satisfying 0.60.ltoreq.X1.ltoreq.0.70 is formed as said first
ridge-shaped layer and a layer having an aluminum composition ratio
X2 of X2.ltoreq.X1 as said second ridge-shaped layer.
16. A method of producing a semiconductor light emitting device as
set forth in claim 14, wherein in the step of forming said second
cladding layer, a layer having an aluminum composition ratio X1 of
0.70 is formed as said first ridge-shaped layer and a layer having
an aluminum composition ratio X2 of 0.65 is formed as said second
ridge-shaped layer.
17. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, said first ridge-shaped layer is formed to have a
film thickness of 50 to 400 nm.
18. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, a sum of a film thickness of a portion excepting
said ridge-shaped portion of said second cladding layer and a film
thickness of said first ridge-shaped layer is made to be 750 nm or
smaller.
19. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, an etching stop layer is formed on a boundary face
of a portion excepting said ridge-shaped portion of said second
cladding layer and said first ridge-shaped layer.
20. A method of producing a semiconductor light emitting device as
set forth in claim 19, wherein in the step of processing said
ridge-shaped portion as the current narrowing structure at the part
of said second cladding layer, the part of said second cladding
layer is processed to be said ridge-shaped portion by etching which
stops at said etching stop layer.
21. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein said first cladding layer, said
active layer and said second cladding layer are formed by an
AlGaInP-based material.
22. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein said first cladding layer, said
active layer and said second cladding layer are formed by an
AlGaN-based material.
23. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, a layer having a same refractive index as that of a
portion excepting said ridge-shaped portion of said second cladding
layer is formed as said first ridge-shaped layer.
24. A method of producing a semiconductor light emitting device as
set forth in claim 13, wherein in the step of forming said second
cladding layer, a layer having a lower refractive index than that
of a portion excepting said ridge-shaped portion of said second
cladding layer is formed as said first ridge-shaped layer.
25. A method of producing a semiconductor light emitting device as
set forth in claim 24, wherein in the step of forming said second
cladding layer, a layer having an aluminum composition ratio of
0.68 is formed as a portion excepting said ridge-shaped portion of
said second cladding layer and a layer having an aluminum
composition ratio of 0.75 to 0.80 is formed as said first
ridge-shaped layer.
Description
TECHNICAL FIELD
[0001] The present invention relates to a semiconductor light
emitting device and a method of producing the same, and
particularly relates to a semiconductor light emitting device with
an improved beam shape and a method of producing the same.
BACKGROUND ART
[0002] A semiconductor laser and other semiconductor light emitting
device are used, for example, as a CD (compact disc) and DVD
(digital versatile disc), furthermore, a light source of an optical
pickup device of next-generation optical disc devices and a light
source of other apparatuses in a variety of fields.
[0003] As the above semiconductor light emitting device, for
example, a semiconductor laser made by an AlGaInP-based material is
disclosed in the non-patent article 1.
[0004] FIG. 1A is a sectional view of the semiconductor laser
explained above.
[0005] For example, an n-type cladding layer 111 formed by an
AlGaInP layer, an active layer 112, a p-type cladding layer 117
formed by AlGaInP layers (113 and 115) and a p-type cap layer 118
formed by a GaAs layer are formed by being stacked on an n-type
substrate 110 via a not shown n-type buffer layer.
[0006] An etching stop layer 114 of a GaInP layer is formed on a
boundary face of the AlGaInP layer 113 and the AlGaInP layer 115,
and a portion from a surface of the p-type cap layer 118 to the
AlGaInP layer 115 is processed to be a ridge (protrusion) shape RD
so as to form a stripe as a current narrowing structure.
[0007] Current block layers 119 are formed on both sides of the
ridge shape RD and, furthermore, a p-electrode 120 is formed to be
connected to the p-type cap layer 118 and an n-electrode 121 is
formed to be connected to the n-type substrate 110.
[0008] FIG. 1B is a view of a bandgap profile of a section along
x.sub.1-x.sub.2 in FIG. 1A.
[0009] It shows a bandgap of each of the n-type cladding layer 111,
active layer 112, AlGaInP layer 113, etching stop layer 114 and
AlGaInP layer 115.
[0010] For example, a composition ratio of aluminum in the n-type
cladding layer 111 is 0.65, while that in both of the AlGaInP
layers (113 AND 115) is 0.70 and p-type cladding layers are
configured to have a higher bandgap than that of the n-type
cladding layer 111.
[0011] In the above semiconductor laser, to adjust an aspect ratio
of the laser beam and bring the beam shape close to a circular
shape is one of significant tasks.
[0012] The beam shape largely depends on a refractive index of each
layer composing the semiconductor laser.
[0013] On the other hand, in the conventional semiconductor laser
explained above, a variety of attempts have been made to improve
the internal quantum efficiency and two leakage currents are
required to be minimum.
[0014] A first leakage current is a lateral direction leakage
current I.sub.Lx, which leaks excessively in the X-direction
parallel to a hetero junction in the sectional view in FIG. 1. A
second leakage current is a longitudinal direction leakage current
I.sub.Ly called an overflow, wherein electrons leak in the
Y-direction from the active layer to the p-cladding layer.
[0015] There is a method of controlling the I.sub.Lx by making a
thickness of the AlGaInP layer 113 in FIG. 1 thin, however, it is
actually difficult to make the AlGaInP layer 113 thin by
controlling to 300 nm or thinner.
[0016] For example, a difference of an effective refractive index
N.sub.eff1 at the center of the ridge stripe and an effective
refraction index N.sub.eff2 outside of the ridge stripe becomes
large, light confinement in the X-direction intensifies, a photon
distribution at the center in the X-direction is maximized, and
electron-hole consumption increases to be short in supply. This is
called hole-burning of carriers and photons are unable to be
supplied with electron holes to maintain the mode at this time, so
that they tend to shift to a mode of receiving the supply. This
phenomenon leads to a change of electron-light conversion
efficiency thereof, and linearity of the light output--current
(L-I) characteristic is deteriorated, which is observed as a
phenomenon called kink.
[0017] Also, in the conventional semiconductor laser explained
above, electrons leak as a longitudinal direction leakage current
I.sub.Ly from the active layer to the p-type cladding layer due to
the thermal electron energy when at a high temperature and
deterioration of the L-I characteristics is caused.
[0018] As a nature of countermeasures thereof, a method of
heightening a height of an energetic barrier sensed by electrons
belonging to a r-band and a method of improving concentration of a
p-type impurity of the cladding layer have been general. At this
time, it is known that a drift current of an electron group
belonging to an X-band increases when the AlGaInP layer 113 is made
thinner as a significant task (refer to the non-patent article
1).
[0019] This can be confirmed also by an experiment and the AlGaInP
layer 113 cannot be made very thin, so that a method of controlling
the leakage current I.sub.Lx in the X-direction explained above
cannot be used.
[0020] Non-Patent Article 1: Numerical Simulation of Semiconductor
Optoelectronic Devices, proceedings, MD4, L39-40
[0021] Non-Patent Article 2: IEEE JQE, vol. 38, No. 3, March 2002,
L285.
DISCLOSURE OF THE INVENTION
[0022] Problem to be Solved by the Invention
[0023] A problem to be solved is a point that it is difficult to
improve an aspect ratio of a laser beam to make it close to a
circular shape in a semiconductor laser having the configuration
shown in FIG. 1.
[0024] Means to Solve the Problem
[0025] A semiconductor light emitting device of the present
invention comprises a substrate; a first conductive type first
cladding layer formed on the substrate; an active layer formed on
the first cladding layer; and a second conductive type second
cladding layer formed on the active layer, a part thereof having a
ridge-shaped portion as a current narrowing structure; wherein said
ridge-shaped portion of the second cladding layer includes a first
ridge-shaped layer on the side close to the active layer and having
a high bandgap and a second ridge-shaped layer on the side distant
from the active layer and having a low bandgap.
[0026] In the above semiconductor light emitting device, a first
conductive type first cladding layer, an active layer and a second
conductive type second cladding layer having a ridge-shaped portion
as a current narrowing structure are stacked on a substrate. The
second cladding layer of the ridge-shaped portion is configured to
include a first ridge-shaped layer on the side close to the active
layer and having a high bandgap and a second ridge-shaped layer on
the side distant from the active layer and having a low
bandgap.
[0027] Also, a method of producing a semiconductor light emitting
device of the present invention includes a step of forming at least
a first conductive type first cladding layer, an active layer and a
second conductive type second cladding layer by stacking on a
substrate by an epitaxial growth method; and a step of processing a
ridge-shaped portion as a current narrowing structure at a part of
the second cladding layer; wherein, in the step of forming the
second cladding layer, a portion to be said ridge-shaped portion is
formed to include a first ridge-shaped layer on the side close to
the active layer and having a high bandgap and a second
ridge-shaped layer on the side distant from the active layer and
having a low bandgap.
[0028] In the above method of producing a semiconductor light
emitting device of the present invention, at least a first
conductive type first cladding layer, an active layer and a second
conductive type second cladding layer are formed by being stacked
on a substrate by an epitaxial growth method and, next, a part of
the second cladding layer is processed to be a ridge-shaped portion
as a current narrowing structure.
[0029] Here, when forming the second cladding layer, the portion to
be the ridge-shaped portion is formed to include a first
ridge-shaped layer on the side close to the active layer and having
a high bandgap and a second ridge-shaped layer on the side distant
from the active layer and having a low bandgap.
EFFECT OF THE INVENTION
[0030] The semiconductor light emitting device of the present
invention has the configuration that the ridge-shaped portion of
the second cladding layer includes a high bandgap layer and a low
bandgap layer, consequently, the configuration that the
ridge-shaped portion of the second cladding layer includes a layer
with a low refractive index and a layer with a high refractive
index is attained, so that a refractive index profile to affect a
beam shape of an emitted light can become adjustable and the aspect
ratio of the beam can be improved to be close to a circular
shape.
[0031] According to the method of producing the semiconductor light
emitting device of the present invention, the ridge-shaped portion
of the second cladding layer is formed to include a high bandgap
layer and a low bandgap layer, so that a refractive index profile
to affect a beam shape of an emitted light can become adjustable
and the aspect ratio of the beam can be improved to be close to a
circular shape.
BRIEF DESCRIPTION OF DRAWINGS
[0032] [FIG. 1] FIG. 1A is a sectional view of a semiconductor
laser as a semiconductor light emitting device according to a
conventional example, and FIG. 1B is a view of a bandgap profile on
a section along x.sub.1-x.sub.2 in FIG. 1A.
[0033] [FIG. 2] FIG. 2A is a sectional view of a semiconductor
laser as a semiconductor light emitting device according to a first
embodiment of the present invention, and FIG. 2B is a bandgap
profile on a section along x.sub.1-x.sub.2 in FIG. 2A.
[0034] [FIG. 3] FIG. 3 is a schematic view for explaining an effect
of reducing drift electrons in the first embodiment of the present
invention.
[0035] [FIG. 4] FIG. 4 is a view showing a result of measuring a
threshold current of a semiconductor laser of an example and a
comparative example in example 1.
[0036] [FIG. 5] FIG. 5 is a view showing results of measuring
.theta..perp. of a semiconductor laser of an example and a
comparative example in example 2.
[0037] [FIG. 6] FIG. 6 is a view showing a result of measuring a
differential efficiency of a semiconductor laser of an example and
a comparative example in example 3.
[0038] [FIG. 7] FIG. 7 is a view showing a result of measuring a
kink level of a semiconductor laser of an example and a comparative
example in example 4.
[0039] [FIG. 8] FIG. 8 is a view wherein a reduction rate KSEp of a
differential coefficient is plotted with respect to a half width
.theta.// of a far-field pattern.
[0040] [FIG. 9] FIG. 9A is a sectional view of a semiconductor
laser as a semiconductor light emitting device according to a
second embodiment of the present invention, and FIG. 9B is a view
showing a bandgap profile on a section along x.sub.1-x.sub.2 in
FIG. 9A.
EXPLANATION OF REFERENCES
[0041] 10 . . . n-type substrate, 11 . . . n-type cladding layer
(first cladding layer), 12 . . . active layer, 13 . . . d2 layer,
14 . . . etching stop layer, 15 . . . d2' layer (first ridge-shaped
layer), 16 . . . second ridge-shaped layer, 17 . . . p-type
cladding layer (second cladding layer), 18 . . . p-type cap layer,
19 . . . current block layer, 20 . . . p-electrode, 21 . . .
n-electrode, 110 . . . n-type substrate, 111 . . . n-type cladding
layer, 112 . . . active layer, 113 . . . AlGaInP layer p-type
cladding layer, 114 . . . etching stop layer, 115 . . . AlGaInP
layer, 117 . . . p-type cladding layer, 118 . . . p-type cap layer,
119 . . . current block layer, 120 . . . p-electrode, 121 . . .
n-electrode
BEST MODE FOR CARRYING OUT THE INVENTION
[0042] Below, embodiments of a semiconductor light emitting device
of the present invention will be explained with reference to the
drawings.
First Embodiment
[0043] FIG. 2A is a sectional view of a semiconductor laser as a
semiconductor light emitting device according to the present
embodiment.
[0044] For example, on an n-type substrate 10, an n-type cladding
layer (first cladding layer) 11 formed by an AlGaInP layer, an
active layer having the multiquantum well structure, a d2 layer 13
formed by an AlGaInP layer, an etching stop layer 14 formed by a
GaInP layer, a d2' layer (first ridge-shaped layer) 15 formed by an
AlGaInP layer and a second ridge-shaped layer 16 formed by an
AlGaInP layer are stacked via a not shown n-type buffer layer,
wherein a portion from the d2 layer 13 to the second ridge-shaped
layer 16 becomes a p-type cladding layer (second cladding layer)
17. Furthermore, a p-type cap layer 18 formed by a GaAs layer is
formed on the second ridge-shaped layer 16.
[0045] Also, a portion from a surface of the p-type cap layer 18 to
the AlGaInP layer 15 is processed to be a ridge (protrusion) shape
RD to form a stripe having a current narrowing structure, and a
current block layer 19 formed, for example, by AlInP, etc. is
formed on both sides of the ridge shape RD.
[0046] Also, a p-electrode 20 is formed to be connected to the
p-type cap layer 18, and an n-electrode 21 is formed to be
connected to the n-type substrate 10.
[0047] FIG. 2B is a bandgap profile on the section along
x.sub.1-x.sub.2 in FIG. 2A.
[0048] It shows a bandgap of each of the n-type cladding layer 11,
active layer 12, d2 layer 13, etching stop layer 14, d2' layer
(first ridge-shaped layer) 15 and second ridge-shaped layer 16.
Here, the height of the bandgap corresponds to the height of the
composition ratio of aluminum, and the higher the composition ratio
of aluminum is, the higher the bandgap becomes.
[0049] For example, a composition ratio of aluminum of the n-type
cladding layer 11 is 0.65, while in the p-type cladding layer,
those of the d2 layer 13 and d2' layer (first ridge-shaped layer)
15 are 0.70 and that of the second ridge-shaped layer 16 is 0.65.
Namely, in the n-type cladding layer 11 and the p-type cladding
layer 17, the profile is, for example, bandgaps of the n-type
cladding layer 11 and the second ridge-shaped layer 16 are low and
bandgaps of the d2 layer and the d2' layer (first ridge-shaped
layer) 15 are high.
[0050] As explained above, in the semiconductor laser of the
present embodiment, the ridge-shaped portion (d2' layer (first
ridge-shaped layer) 15 and second ridge-shaped layer 16) of the
p-type cladding layer (second cladding layer) 17 is configured to
include the d2' layer (first ridge-shaped layer) 15 with a high
bandgap on the side close to the active layer 12 and the second
ridge-shaped layer 16 with a low bandgap on the side distant from
the active layer 12.
[0051] Also, in the p-type cladding layer 17, the portion of the d2
layer 13 and the d2' (first ridge-shaped layer) 15 is configured to
have a higher bandgap than that of the n-type cladding layer
11.
[0052] Also, the height of the aluminum composition ratio
corresponds to the height of the refractive index, and the higher
the aluminum composition ratio is, the lower the refraction ratio
is. Accordingly, the profile is, for example, refraction indexes of
the n-type cladding layer 11 and the second ridge-shaped layer 16
are high and those of the d2 layer 13 and the d2' (first
ridge-shaped layer) 15 are low, that is, the d2' layer (first
ridge-shaped layer) 15 is configured to be formed by a layer having
the same refraction index as that of the d2 layer 13 as a portion
excepting the ridge-shaped portion of the p-type cladding layer
(second cladding layer) 17.
[0053] In a semiconductor laser as the semiconductor light emitting
device according to the above present embodiment, as a result of
applying a predetermined voltage to the p-electrode 20 and the
n-electrode 21, a laser beam having a wavelength of, for example,
650 nm band is emitted from the laser beam emitting portion in the
direction parallel to the substrate.
[0054] The above semiconductor laser may be an index guide and self
pulsation type, etc. by controlling a depth and shape, etc. of the
ridge.
[0055] The semiconductor light emitting device according to the
above explained present embodiment has the configuration that the
ridge-shaped portion of the p-type cladding layer (second
ridge-shaped layer) includes a high bandgap layer and a low bandgap
layer, consequently, the configuration that the ridge-shaped
portion of the second cladding layer includes a layer with a low
refractive index and a layer with a high refractive index is
attained, so that a refractive index profile to affect a beam shape
of an emitted light can become adjustable, for example, a half
width (.theta..perp.) of a far-field pattern beam in the vertical
direction with respect to the hetero junction becomes small, and
the aspect ratio of the beam can be improved to generate a beam
pattern closer to a circular shape.
[0056] In the semiconductor laser of the present embodiment, it is
preferable that a composition ratio X1 of aluminum in the d2' layer
(first ridge-shaped layer) 15 satisfies 0.60.ltoreq.X1.ltoreq.0.70
and a composition ratio X2 of aluminum in the second ridge-shaped
layer 16 satisfies X2.ltoreq.X1.
[0057] By attaining this configuration, a film thickness of the d2
layer 13 as a portion excepting the ridge-shaped portion of the
p-cladding (second cladding) layer having a high aluminum
composition ratio can be made as thin as 50 to 350 nm, as a result,
the current I.sub.Lx leaking excessively in the direction parallel
to the hetero junction can be reduced.
[0058] As explained above, in the configuration of the present
embodiment, as a result that the ridge portion is configured to
include the d2' layer (first ridge-shaped layer) 15 as a low
refractive index layer and the second ridge-shaped layer 16 as a
high refractive index layer, even when the d2 layer 13 is made thin
as 50 to 350 nm, a threshold current (threshold carrier density) of
the semiconductor laser can be reduced, an overflow of electrons
from the active layer to the p-side, which has been a problem, can
be suppressed, and the differential efficiency and kink level can
be improved.
[0059] In the present embodiment, to correct the thinness of the d2
layer 13, the d2' layer (first ridge-shaped layer) 15 having a high
Al composition (0.60.ltoreq.X1 0.70) is introduced, and the
thickness can be made as thick as 50 to 400 nm.
[0060] Theoretically, electrons overflowed from the active layer 12
may pass through the d2 layer 13 via an X-band to rejoin at the
etching stop layer 14, while, a reduction of a threshold current
value and an improvement effect of a temperature characteristic,
etc. were observed due to an effect of the d2' layer (first
ridge-shaped layer) 15 by way of experiment.
[0061] In the case where the d2' layer (first ridge-shaped layer)
15 having a high aluminum composition ratio is not formed, when the
d2 layer 13 as a portion excepting the ridge-shaped portion of the
p-cladding layer (second cladding layer) is made thin in order to
aim the above effects, it is liable that an electron group
belonging to the X-band passes through the d2 layer, acts as drift
electrons, leaks to the p-type cladding layer and adversely leads
to a deterioration of the temperature characteristic (refer to the
non-patent article 2).
[0062] FIG. 3 is a schematic view for explaining an effect of
reducing drift electrons in the present embodiment.
[0063] In the present embodiment, the ridge-shaped portion of the
p-type cladding layer (second cladding layer) 17 is composed of the
d2' layer (first ridge-shaped layer) 15 having a high bandgap and
the second ridge-shaped layer 16 having a low bandgap. The d2'
(first ridge-shaped layer) 15 is provided not to contact with a SCH
(Separate Confinement Hetero-structure) guide layer of the active
layer 12 and the d2 layer 13 and the etching stop layer 14 are
provided between them, and it is confirmed by way of experiment
that the effect of suppressing the drift electrons enhances as the
thickness increases.
[0064] In an AlGaInP-based high-energy laser, a stripe width of a
lower side of the trapezoidal ridge shape RD, a sectional view of
which is as shown in FIG. 2A, has to be as narrow as 2.5 .mu.m or
less to improve the kink level. However, it is technically
difficult to make the ridge shape upright and, when the stripe
width of the lower side becomes narrow, the upper side of the ridge
trapezoid becomes extremely narrow to cause a new disadvantage that
the resistance becomes high.
[0065] In the semiconductor laser structure according to the
present embodiment, the d2' layer (first ridge-shaped layer) 15 in
the figure has a high Al composition on average than that in the
second ridge-shaped layer 16 formed above it. Therefore, in a wet
etching step for producing the ridge shape in the figure, an
etching rate of the d2' layer (first ridge-shaped layer) 15 becomes
faster than that of the second ridge-shaped layer 16.
[0066] Consequently, etching proceeds faster at the lower portion
of the ridge shape RD, so that the stripe width of the lower side
can be made narrower by 0.2 .mu.m or so comparing with that in the
case of producing the same upper side. Namely, the ridge shape can
be more upright comparing with that in the conventional cases, so
that the kink level improves.
[0067] From the above reason, a film thickness of the d2 layer 13
is preferably 50 to 350 nm or so. When it exceeds 350 nm, the
current ILx leaking excessively in the direction parallel to the
hetero junction increases, which is unfavorable.
[0068] Also, a sum of film thicknesses of the d2 layer 13 and the
d2' (first ridge-shaped layer) 15 is preferably 750 nm or smaller.
When exceeding 750 nm, .theta..perp. of the beam declines.
[0069] Also, a film thickness of the d2' (first ridge-shaped layer)
layer 15 is preferably 50 to 400 nm or so. This is for the sum of
film thicknesses of the d2 layer 13 and the d2' (first ridge-shaped
layer) 15 not to exceed 750 nm as explained above.
EXAMPLE 1
[0070] A semiconductor laser having the configuration shown in FIG.
2 was produced as an example by following the above embodiment, and
a semiconductor laser having the configuration shown in FIG. 1 was
produced as a comparative example. A threshold current was measured
on both of the semiconductor lasers.
[0071] The results are shown in FIG. 4.
[0072] A lower threshold current was obtained in the semiconductor
laser of the example than that of the comparative example.
EXAMPLE 2
[0073] In the same way as in the example 1, a semiconductor laser
as an example and that as a comparative example were produced, a
far-field pattern in the vertical direction with respect to the
hetero junction was observed on both of the semiconductor lasers
and the .theta..perp. was measured.
[0074] The results are shown in FIG. 5.
[0075] A smaller .theta..perp. value was obtained in the
semiconductor laser as the example than that of the comparative
example.
EXAMPLE 3
[0076] In the same way as in the example 1, a semiconductor laser
as an example and that as a comparative example were produced, and
a differential efficiency was measured on both of the semiconductor
lasers.
[0077] The results are shown in FIG. 6.
[0078] A larger differential efficiency value was obtained in the
semiconductor laser as the example than that of the comparative
example.
EXAMPLE 4
[0079] In the same way as in the example 1, a semiconductor laser
as an example and that as a comparative example were produced, a
kink level (100 ns, 70.degree. C.) was measured on both of the
semiconductor lasers.
[0080] The results are shown in FIG. 7.
[0081] The kink level was improved in the semiconductor laser as
the example comparing with that in the comparative example.
EXAMPLE 5
[0082] In the same way as in the example 1, a semiconductor laser
as an example and that as a comparative example were produced, and
a reduction rate KSEp of a differential coefficient of the L-I
curve and a half width .theta.// of a far-field pattern as an
indication of light confinement of a light in the X-direction were
measured on both of the semiconductor lasers. The larger the KSEp
value is, the higher the tortuosity of L-I (arising of kink)
is.
[0083] FIG. 8 is a view wherein reduction rates KSEp of
differential coefficients is plotted with respect to the half width
.theta.// (output of 5 mW) of a far-field pattern.
[0084] In the comparative example, when the half width .theta.// of
the far-field pattern is large, kink easily arises along with the
hole burning effect.
[0085] In the example, the kink level does not decline even when
the half width .theta.// of the far-field pattern becomes
large.
[0086] This also contributes to an improvement of the aspect ratio
of the beam to generate a more circular beam pattern as explained
above, and the merit is significant in an optical disk
application.
EXAMPLE 6
[0087] In the same way as in the example 1, a semiconductor laser
as an example and that as a comparative example were produced, and
an operation current value during an operation at a high
temperature was measured on both of the semiconductor laser.
[0088] The semiconductor laser as the example exhibited a smaller
operation current value in high temperature operation comparing
with that of the comparative example.
[0089] A method of producing the semiconductor laser according to
the present embodiment as above will be explained.
[0090] For example, by using an epitaxial growth method, such as an
organic metal vapor epitaxial growth method (MOVPE), a not shown
buffer layer, the n-type cladding layer (first cladding layer) 11
formed by an AlGaInP layer, the active layer 12, the d2 layer 13
formed by an AlGaInP layer, the etching stop layer 14 formed by a
GaInP layer, the d2' layer (first ridge-shaped layer) 15 formed by
an AlGaInP layer, the second ridge-shaped layer 16 formed by an
AlGaInP layer and the p-type cap layer 18 formed by a GaAs layer
are stacked in order on the n-type substrate 10. Here, a portion
from the d2 layer 13 to the second ridge-shaped layer 16 becomes
the p-type cladding layer (second cladding layer) 17.
[0091] Here, film formation is performed, so that, for example, a
composition ratio of aluminum in the n-type cladding layer 11 is
0.65, those of the d2 layer 13 and d2' layer (first ridge-shaped
layer) 15 as p-type cladding layers are 0.70, and that of the
second ridge-shaped layer 16 is 0.65.
[0092] Namely, in a step of forming the p-type cladding layer
(second cladding layer) 17, a layer having the same refraction
index as that of a portion excepting the ridge-shaped portion of
the second cladding layer is formed as the d2' (first ridge-shaped
layer) 15.
[0093] Next, for example, AlInP, etc. is stacked allover the
surface to form a current block layer 19, and a contact opening is
formed, so that the p-type cap layer 18 is exposed.
[0094] Next, the p-electrode 20 made by Ti/Pt/Au, etc. is formed to
be connected to the p-type cap layer 18, and the n-electrode 21
made by AuGe/Ni/Au, etc. is formed to be connected to the n-type
substrate 10.
[0095] After that, through a pelletizing step, a desired
semiconductor laser as shown in FIG. 2A can be obtained.
[0096] In the method of producing the semiconductor light emitting
device of the present embodiment, since the ridge-shaped portion of
the second cladding layer is formed to include a layer with a high
bandgap and a layer with a low bandgap, the configuration that the
ridge-shaped portion of the second cladding layer includes a layer
with a low refractive index and a layer with a high refractive
index is attained, so that a refractive index profile to affect a
beam shape of an emitted light can become adjustable and the aspect
ratio of the beam can be improved to be close to a circular
shape.
[0097] In the above embodiment, an AlGaInP-based semiconductor
light emitting device is explained, but the present embodiment is
not limited to that and may be applied to an AlGaN-based
semiconductor light emitting device.
[0098] The layer composition and the configuration can be same as
those in the AlGaInP-based case in FIG. 2A. In this case, it is
preferable that an aluminum composition ratio X1 of the d2' layer
(first ridge-shaped layer) is 0.05.ltoreq.X1.ltoreq.0.20 and an
aluminum composition ratio of layers, such as the second
ridge-shaped layer, other than the d2' layer (first ridge-shaped
layer) is X2.ltoreq.0.20. As a result, the same effects as those in
the case of the AlGaInP-based semiconductor light emitting device
can be obtained.
Second Embodiment
[0099] FIG. 9A is a sectional view of a semiconductor laser as a
semiconductor light emitting device according to the present
embodiment.
[0100] A semiconductor laser according to the present embodiment
has the same configuration as that in the first embodiment. For
example, on an n-type substrate 10, an n-type cladding layer (first
cladding layer) 11 formed by an AlGaInP layer, an active layer 12
having the multiquantum well structure, a d2 layer 13 formed by an
AlGaInP layer, an etching stop layer 14 formed by a GaInP layer, a
d2' layer (first ridge-shaped layer) 15 formed by an AlGaInP layer
and a second ridge-shaped layer 16 formed by an AlGaInP layer are
stacked via a not shown n-type buffer layer, wherein a portion from
the d2 layer 13 to the second ridge-shaped layer 16 becomes a
p-type cladding layer (second cladding layer) 17. Furthermore, a
p-type cap layer 18 formed by a GaAs layer is formed on the second
ridge-shaped layer 16.
[0101] Also, a portion from a surface of the p-type cap layer 18 to
the AlGaInP layer 15 is processed to be a ridge (protrusion) shape
RD to form a stripe having a current narrowing structure, and a
current block layer 19 formed, for example, by AlInP, etc. is
formed on both sides of the ridge shape RD.
[0102] Also, a p-electrode 20 is formed to be connected to the
p-type cap layer 18, and an n-electrode 21 is formed to be
connected to the n-type substrate 10.
[0103] FIG. 9B is a bandgap profile on the section along
x.sub.1-x.sub.2 in FIG. 9A.
[0104] It shows a bandgap of each of the n-type cladding layer 11,
active layer 12, d2 layer 13, etching stop layer 14, d2' layer
(first ridge-shaped layer) 15 and second ridge-shaped layer 16.
Here, the height of the bandgap corresponds to the height of the
aluminum composition ratio, and the higher the aluminum composition
ratio is, the higher the bandgap becomes.
[0105] In the semiconductor laser of the present embodiment, an
aluminum composition ratio X0 of the d2 layer 13, an aluminum
composition ratio X1 of the d2' layer (first ridge-shaped layer)
15, and an aluminum composition ratio X2 of the second ridge-shaped
layer 16 as a ridge-shaped portion other than the d2' layer (first
ridge-shaped layer) 15 satisfy X2<X0<X1. An aluminum
composition ratio of the n-type cladding layer 11 is made equal to
the aluminum composition ratio of the second ridge-shaped layer
16.
[0106] For example, the aluminum composition ratio in the n-type
cladding layer 11 is 0.65, while in the p-type cladding layer, that
in the d2 layer 13 is 0.68, that of the d2' layer (first
ridge-shaped layer) 15 is 0.75 to 0.80 and that in the second
ridge-shaped layer 16 is 0.65.
[0107] Namely, the n-type cladding layer 11 and the p-type cladding
layer 17 has a profile that, for example, a bandgap is low in the
n-type cladding layer 11 and the second ridge-shaped layer 16, a
bandgap is high in the d2 layer 13, and a bandgap in the d2' layer
(first ridge-shaped layer) 15 is still higher.
[0108] The semiconductor layer of the present embodiment is
configured that the ridge-shaped portion (the d2' layer (first
ridge-shaped layer) 15 and second ridge-shaped layer 16) of the
p-type cladding layer (second cladding layer) 17 includes a d2'
layer (first ridge-shaped layer) 15 on the side close to the active
layer 12 and having a high band gap and a second ridge-shaped layer
16 on the side distant from the active layer 12 and having a low
bandgap.
[0109] Also, a portion of the d2 layer 13 and the d2' layer (first
ridge-shaped layer) 15 in the p-type cladding layer 17 is
configured to have a higher bandgap than that of the n-type
cladding layer 11.
[0110] Also, the height of the refraction index corresponds to the
height of the aluminum composition ratio, and the higher the
aluminum composition ratio, the lower the refractive index
becomes.
[0111] Accordingly, with the above aluminum composition profile, a
refraction index profile, that a refraction index of the n-type
cladding layer 11 and that of the second ridge-shaped layer 16 are
high, that of the d2 layer 13 is low and that of the d2' layer
(first ridge-shaped layer) 15 is still lower, is obtained in the
n-type cladding layer 11 and the p-type cladding layer 17.
[0112] Accordingly, the d2' layer (first ridge-shaped layer) 15 is
configured to be composed of a layer with a lower refractive index
than that of the d2 layer 13 as a portion excepting the
ridge-shaped portion of the p-type cladding layer (second cladding
layer) 17.
[0113] Other than the above, the semiconductor laser of the present
embodiment is substantially the same as that in the first
embodiment.
[0114] In the semiconductor laser as a semiconductor light emitting
device according to the present embodiment explained above, by
applying a predetermined voltage to the p-electrode 20 and the
n-electrode 21, a laser beam having a wavelength of, for example,
650 nm band is emitted from the layer light emitting portion in the
direction parallel to the substrate.
[0115] The above semiconductor laser may become an index guide and
self pulsation type, etc. by controlling a depth and shape of the
ridge.
[0116] The semiconductor light emitting device according to the
present embodiment as above has the configuration that the
ridge-shaped portion of the p-type cladding layer (second cladding
layer) includes a high bandgap layer and a low bandgap layer,
consequently, the configuration that the ridge-shaped portion of
the second cladding layer includes a layer with a low refractive
index and a layer with a high refractive index is attained, so that
the refractive index profile to affect on the beam shape of the
emitted light becomes adjustable, for example, the half width
(.theta..perp.) of the far-field pattern beam in the vertical
direction with respect to the hetero junction becomes small, and
the aspect ratio of the beam can be improved to generate a more
circular beam pattern.
[0117] Particularly, as explained above, when with the refractive
index profile that the refractive index of the n-type cladding
layer 11 and that of the second ridge-shaped layer 16 are high,
that of the d2 layer 13 is low, and that of the d2' layer (first
ridge-shaped layer) 15 is still lower, a light distribution in the
laser longitudinal direction can be designed at a higher degree of
freedom, moreover, by adjusting the aluminum composition ratio of
the second ridge-shaped layer 16 and film thicknesses of the d2
layer 13 and the d2' layer (first ridge-shaped layer) 15 and
adjusting aluminum composition ratios of the di2 layer 13 and the
d2' layer (first ridge-shaped layer) 15, the light distribution can
be optimized and a spot of a laser beam to be emitted can be made
closer to a perfect circle.
[0118] In the semiconductor laser according to the present
embodiment, it is also preferable, as in the same way as in the
first embodiment, a film thickness of the d2 layer 13 is 50 to 350
nm or so, a film thickness of the d2' layer (first ridge-shaped
layer) 15 is 50 to 400 nm or so, and a sum of film thicknesses of
the d2 layer 13 and the d2' layer (first ridge-shaped layer) 15 is
750 nm or smaller.
[0119] The semiconductor laser according to the present embodiment
is capable of attaining the configuration that the aluminum
composition ratio of the d2' layer (first ridge-shaped layer) 15 is
furthermore higher than that in the first embodiment. Here, the
higher the aluminum composition is, the faster the etching rate of
processing a ridge shape, so that the etching rate ratio thereof to
the etching stop layer 14 can be increased and it is possible to
make the ridge shape more upright when processing the ridge shape
comparing with that in the first embodiment, so that the kink level
improves. Also, etching unevenness on a wafer surface of the
cladding can be suppressed.
[0120] The semiconductor laser according to the present embodiment
can be produced in the same way as that in the first embodiment by
forming as the first ridge-shaped layer a layer having a lower
refractive index than that of a portion excepting the ridge-shaped
portion of the second cladding layer in the step of forming the
second cladding layer.
[0121] The present invention is not limited to the above
explanations.
[0122] For example, the present invention can be applied to an
AlGaAs-based semiconductor light emitting device other than an
AlGaInP-based and AlGaN-based semiconductor light emitting
devices.
[0123] Other than that, a variety of modifications may be made
within the scope of the present invention:
INDUSTRIAL APPLICABILITY
[0124] The semiconductor light emitting device of the present
invention can be applied as a CD and a DVD, moreover, a light
source of an optical pickup device of a next-generation optical
disc apparatus and a light source of other apparatuses in a variety
of fields.
[0125] The method of producing the semiconductor light emitting
device of the present invention can be applied as a method for
producing a CD and a DVD, moreover, a light source of an optical
pickup device of a next-generation optical disc apparatus and a
light source of other apparatuses.
* * * * *